1. Field of the Invention
The present invention relates to a washing technology. More particularly, the present invention relates to a washing solution and method of washing away paint residue when a color to be coated is different to a color previously coated, (hereinafter, color-change washing). Particularly, this washing solution and method can be advantageously used when automobile bodies and other articles are to be successively coated with different color paints by a rotary type electrostatic spray coating device.
2. Description of the Related Art
As is well-known, an electrostatic spray coating technology is widely used in the spray coating of automobile bodies and other products, and typically, a rotary type electrostatic spray coating device is used in such a technology or process. In this type of spray coating device, if a color-change coating is carried out, i.e., if two or more paints of different colors are successively coated on the same article to provide an attractive appearance, conventionally a spray head of the coating device is washed with a suitable washing solution or thinner after completion of a coating using a paint of a first color and before a next coating using a paint of a second color. This washing step is referred to herein as color-change washing, and effectively removes a film or particles of the first color paint from the spray head to which the paint is adhered, to thereby prevent contamination of the coating device due to the paint used. Generally, the washing solution or thinner used in this color-change washing is a mixed solvent and comprises a true solvent, an assistant solvent, a diluent and, optionally, other components or additives Examples of the true solvents conventionally used in this field include low-boiling ketones having a boiling point of less than 100.degree. C., for example, acetone (b.p. 81.degree. C.), methyl ethyl ketone (b.p. 79.6.degree. C.) and the like; and medium-boiling esters and glycol ethers having a boiling point of from 100.degree. C. to less than 150.degree. C. Typical examples of the assistant solvents used in combination with said true solvents include alcohols such as methyl alcohol, ethyl alcohol and the like. In addition, typical examples of the diluents used include medium-boiling hydrocarbons having a boiling point of from 100.degree. C. to less than 150.degree. C., for example, toluene (b.p. 110.7.degree. C.) and xylene (b.p. ca 140.degree. C.). These solvent components may be mixed indifferent proportions in accordance with various factors such as the requirements defined in the U.S. E.P.A. Regulation related to the use of organic solvents and the fields of utilization of the washing solution.
Bodies of automobiles are often coated with a metallic paint having aluminum or other metal powders incorporated therein, to improve the appearance of the coated bodies. For this metallic coating, a non-aqueous dispersion paint, also referred to as an NAD paint, is used in combination with a titanium dioxide pigment incorporated paint. The NAD paint is a relatively low viscosity dispersion or suspension of a high concentration of polymeric resins such as acrylic resin having a relatively high molecular weight, in a suitable non-polar organic solvent, and is used to form a clear coating. Compared with conventional solvent type paints, this paint has many advantages, such as low environmental pollution, saving in raw materials, formation of thicker coatings, and an improved operability of the coating process. The titanium dioxide pigment-containing paint is used to form a white-colored coating, but when the NAD and titanium dioxide pigment-containing paints are used separately for color-change painting in a rotary type electrostatic spray coating device, and the washing solution or thinner is used for the color-change washing, a serious problem arises due to the use of this washing solution. Namely, assuming that a solvent selected from the group of true solvents is a good solvent for the NAD paint and is used in the washing solution, the selected solvent can dissolve, swell and thicken the NAD paint during a color-change washing of a spray head and other parts of the coating device with the washing solution, after completion of the NAD coating. As a result, the thickened NAD paint acts as an adhesive and adheres to the spray head, paint nozzles, pipes and other parts of the coating device, to thereby coagulate titanium dioxide pigments therein. The titanium dioxide pigments are derived from the white-colored paint which is sprayed after the color-change washing, and a coagulation of such pigments can eventually cause problems such as a serious contamination and clogging of the coating device.
The contamination and clogging of the coating device is a serious problem, and to solve this problem, the present inventors have created an improved solution for washing the coating device. This washing solution, as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 63-4879, published Jan. 9, 1988, comprises a true solvent an assistant solvent and a diluent as main components, and is characterized in that a good solvent to the NAD paint is not included or is included only in a negligible amount in the washing solution, but an amount of the solvents in which the NAD paint is insoluble is increased. This washing solution ensures that the NAD paint is washed from the coating device during a color-change washing, and thus the problems of contamination and clogging of the coating device are completely resolved.
Further, a recent study revealed that the problem of coagulation of the titanium dioxide pigments in the spray head and paint nozzles in the coating device due to the adhesive property of the thickened NAD paint is also caused because of the boiling point of the solvents used in the washing solution, in addition to the use of the good solvent to the NAD paint as described in the above paragraphs. Namely, the low-boiling Ketones used as the true solvent in the washing solution have a rapid drying property, and therefore, the NAD paint dissolved therein during color-change washing adheres to the spray head and paint nozzles prior to the washing of the same from the coating device. Also, such low-boiling Ketones must be handled with care, because of the danger of a combustion thereof due to a low flash point of the Ketones of less than about 0.degree. C.